Roadway planers, also known as pavement profilers, road milling machines or cold planers, are machines designed for scarifying, removing, mixing or reclaiming, material from the surface of bituminous or concrete roadways and similar surfaces. These machines typically have a plurality of tracks or wheels which support and horizontally transport the machine along the surface of the road to be planed, and have a rotatable planing cylinder that is vertically adjustable with respect to the road surface.
On cold planers that integrate the machine chassis with the planing cylinder, as described in U.S. Pat. No. 4,186,968, issued Feb. 5, 1980, to Robert M. Barton and currently assigned to the assignee of the present invention, or those similar to the cold planer described in U.S. Pat. No. 4,929,121 issued May 29, 1990 to Kevin C. Lent et al. and assigned to the assignee of the present invention, raise or lower the entire chassis to control the depth of cut of the cutting bits into the ground surface. If the cutting bits strike a high density obstruction, such as a manhole cover or railroad track during the planing operation, the bits on the planing cylinder can be damaged or an event known as a "kickback" can occur.
When a kickback event occurs, the planing cylinder on a typical down-cutting machine will attempt to rise up out of the cut. In a similar manner, changes in material density can cause the chassis on an up-cutting machine to also rise up out of the cut. If the cold planer is operating with an automatic grade control system, such as the portable string line system described in U.S. Pat. No. 4,270,801 issued Jun. 2, 1981 to George M. Swisher, Jr. et al, the automatic grade control, sensing that the machine is above the desired grade, will attempt to lower the chassis by retracting the supporting strut members, leaving the machine principally supported on the rotor. In this position, the machine cannot be steered or braked because of insufficient contact between the strut mounted tracks, or wheels, and the ground. In this condition, the operator may not be able to stop, steer, or control undesirable movement of the machine.
It is desirable for the planer operator to raise the planing cylinder above the top of such an obstruction, pass the planing cylinder over the obstruction and then return the planing cylinder to milling the pavement at the depth previously used. Generally, this function is manually performed by the planer operator. However, once the planing cylinder passes over the obstruction and begins milling the pavement, often the milling is at a different depth than before the obstruction. This can affect the smoothness of the new pavement that is applied later.
Therefore, it is desirable to have an automatic obstruction avoidance control system that will return the planing cylinder to milling the same depth of pavement after the obstruction as was being milled before the obstruction. It is also desirable to have a method of controlling the operation of a cold planer so that the milling depth before the obstruction and after the obstruction is the same.
The present invention is directed to overcoming one or more of the problems as set forth above.